Periostin is an extracellular matrix protein and is composed of an EMI region, an R1 region, an R2 region, an R3 region, an R4 region, and a C-terminal region in this order from the N-terminus to the C-terminus. One of the present inventors, K. Izuhara, has found that measurement of the expression level of the periostin gene is useful as a method of testing for allergic disease and has accomplished an invention relating to a method of testing for allergic disease (see Patent Literature 1 and Non Patent Literature 1).
K. Izuhara has also found that measurement of the expression level of the periostin gene is useful as a method of testing for idiopathic interstitial pneumonia (see Patent Literature 2).
Furthermore, there have been disclosed a polyclonal antibody and a monoclonal antibody against OSF2 (periostin) and a diagnostic method using these antibodies (see Patent Literature 3), an immunoassay of a novel osteoblast specific transcription factor named as Osf2/Cbfa1 using an anti-OSF2 (periostin) antibody (see Patent Literature 4), a purified antibody specifically binding to human periostin and a diagnostic assay for investigating, for example, metastasis of breast cancer to bone using the antibody (see Patent Literature 5), and an antibody against periostin having anti-cell adhesion activity and a method of quantitatively measuring periostin using the antibody (see Patent Literature 6).
However, such measurements of periostin useful for testing various diseases have been required to be improved in accuracy, such as sensitivity and specificity, of the measurement for differentiating patients affected with the diseases from healthy subjects and patients affected with other diseases.